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Qt is a cross-platform application development framework, widely used for the development of graphical user interface programs, and, since the release of Qt 4, also used for developing non-GUI programs such as console tools and servers. Qt is most notably used in KDE, Qtopia and OPIE. It is produced by the Norwegian company Trolltech, formerly Quasar Technologies and was later bought by Nokia. Trolltech insiders pronounce Qt as "cute".

The Qt designer used for GUI designing

Qt uses standard C++, but extends the language by providing an additional pre-processor that converts Qt's extensions into pure C++. Qt can also be used by programmers using other languages; bindings exist for Python (PyQt), Ruby, PHP, C, Perl, Pascal, and Java (Jambi). It runs on all major platforms, and has extensive internationalization support. Non-GUI features include SQL database access, XML parsing, thread management, and a unified cross-platform API for file handling.

The first three editions are released under a commercial license which permits closed source development; while the Open Source edition is available under the GPL license, and the LGPL (starting with version 4.5) and additionally under the Q Public License (QPL) for the Qt/X11 version.

In case of the X11 platform, the QPL allows the final application to be licensed under various open source licenses, such as the LGPL or the Artistic license. For the Windows and Mac OS X platforms, the GPL is the only Open Source license available so the applications developed with it must be GPL as well.

All editions support a wide range of compilers, including the GCC C++ compiler. Official support for the Visual Studio suite is, however, restricted to the commercial Qt/Windows edition. The Q../Free project has released several patches which add support for Microsoft Visual Studio and Borland C++ Builder to the open-source version of Qt/Windows.

Trolltech released Qt 4 on June 28, 2005 and introduced five new technologies in the framework:

Tulip A set of template container classes.

Interview A model/view architecture for item views.

Arthur A 2D painting framework.

Scribe A Unicode text renderer with a public API for performing low-level text layout.

MainWindow A modern action-based main window, toolbar, menu, and docking architecture.

Qt 4 is dual-licensed under GPL and proprietary licenses on all supported platforms including Windows (while Qt/Windows 3.3 is only released under a proprietary license).

Qt 4.1, released on December 19, 2005, introduced integrated SVG Tiny support, a PDF backend to Qt's printing system, and a few other features.

Qt 4.2, released on October 4, 2006, introduced native CSS support for widget styling, as well as the QGraphicsView framework for efficient rendering of thousands of 2D objects onscreen, to replace Qt 3.x's QCanvas class.

Haavard Nord and Eirik Chambe-Eng (the original developers of Qt and the CEO and President of Trolltech respectively) began development of "Qt" in 1991, three years before the company was incorporated as Quasar Technologies, then changed the name to Troll Tech, and then to Trolltech.

The toolkit was called Qt because the letter Q looked beautiful in Haavard's Emacs font, and T was inspired by Xt, the X toolkit.

Controversy erupted around 1998 when it became clear that KDE was going to become one of the leading desktop environments for GNU/Linux. As KDE was based on Qt, many people in the open source and free software movements were worried that an essential piece of one of their major operating systems would be proprietary.

This gave rise to two efforts: the Harmony toolkit which sought to duplicate the Qt Toolkit under a free software license and the GNOME desktop that was meant to supplant KDE entirely. The GNOME Desktop uses the GTK+ toolkit which was written for the GIMP, and mainly uses the C programming language.

Until version 1.45, source code for Qt was released under the FreeQt license — which was viewed as not compliant to the open source principle by the Open Source Initiative and Free Software Foundation because while the source was available it did not allow the redistribution of modified versions. With the release of version 2.0 of the toolkit, the license was changed to the QPL, a free software license but one regarded by the Free Software Foundation as incompatible with the GPL. Compromises were sought between KDE and Trolltech wherein Qt would not be able to fall under a more restrictive license than the QPL, even if Trolltech was bought out or went bankrupt. This led to the creation of the KDE Free Qt foundation, which guarantees that Qt would fall under a BSD license should no open source version of Qt be released during 12 months.

The first versions of Qt had only two flavors: Qt/X11 for Unix and Qt/Windows for the Windows platform. The Windows platform was only available under the commercial license. In the end of 2001, Trolltech released Qt 3.0 which added support for the Mac OS X platform. The Mac OS X support was available only in the commercial license, until June 2003, where Trolltech released the version 3.2 with Mac OS X support available under the GPL license.

In 2002 members of the KDE on Cygwin project began porting the GPL licensed Qt/X11 code base to Windows. This was in response to Trolltech's refusal to license Qt/Windows under the GPL on the grounds that Windows was not an open-source platform. The project achieved reasonable success although it never reached production quality. Qt/Windows 4 was released under the GPL by Trolltech in June 2005. Qt4 now supports the same set of platforms in the Open Source editions as in the commercial edition.

Qt uses its own paint engine and controls. This makes the work of porting to other platforms easier because very few classes in Qt depended really on the target platform. Qt used to emulate the native look of its intended platforms, which occasionally led to slight discrepancies where that emulation wasn't perfect. This, however, no longer applies because the latest versions of Qt use the native styles API of the different platforms to draw the Qt controls.

Known as the moc, this is a tool that one must run on the sources of a Qt program prior to compiling it. The tool will generate "Meta Information" about the classes used in the program. This meta information is used by Qt to provide programming features not available in C++: introspection and the signals and slots|signal/slot system.

The use of an additional tool has been criticised by part of the C++ community, stating that Qt programming is making a mockery of C++. In particular, the choice of an implementation based on macros has been criticized for its absence of type safety. Some people confused the macros with new C++ keywords, and criticized the pollution of the namespace. This is viewed by Trolltech as a necessary trade-off to provide introspection and dynamically generated slots or signals. Further, when Qt 1.x was released, consistency between compiler template implementations could not be relied upon.

The use of moc allows for a very flexible signal/slot messaging system, so that multiple signals can connect to multiple slots, to other signals, or to slots on different threads, in a "callback safe" manner.

Mandel is an interactive, multiplatform and multilingual program by Wolf Jung for drawing the Mandelbrot set and Julia sets, and for illustrating and researching their mathematical properties. It's graphical user interface is based on Qt. It is available on Linux, Unix, Windows, and Mac. The source code is available under the GNU General Public License.